A case provided with an information carrier consisting of a tape one side of which has a structure carrying optically readable information as well as an apparatus for writing, reading or erasing such informtion.
The present invention relates to a case with an information carrier consisting of a tape one side of which has a structure carrying optically readable information, which case is a dustproof housing enclosing two reels accomodating said moving tape, said housing being provided with a window which is positioned such that a part of the tape present between the two reels is visible through this window. The case is furthermore provided with pressing means for guiding the tape in such a manner that the part of said tape visible through said window is kept in accurately fixed position with respect to this window.
The present invention furthermore relates to an apparatus for writing, reading or erasing information on such an information carrier in such a case.
Such a case provided with an information carrier for optically readable information is known from the British Patent Specification No. 1,363,799 which discloses a film cassette provided with two reels accomodating a film strip. This film strip may be exposed, developed and projected while it is not required to open the cassette and to remove the film therefrom. To that effect the cassette is among others provided with two windows. A part of this film strip present between said two reels is located behind one of the windows and is visible through this window from the outside. Through this window the film strip may be exposed. A second window serves for projecting images present on said film strip. Through this window light from a projection lamp is transmitted into the cassette and falls upon incidence through a first face of a prism mounted in said cassette. In this prism is reflected completely by a second face of this prism the light and leaves said prism through a third face of this prism whereafter it is transmitted through said film strip and said first window to the exterior. Hence the images present on said film strip are projected by means of passing light entering the cassette through a window, falling upon the film strip after reflection by a reflecting means and leaving said cassette again after passing said film strip through the other window. The projected part of the film strip is present between the inner side of one of the windows and one face of a prism fixedly mounted behind said window. Upon exposure the part of the film strip to be exposed is also present in the same position.
It is known that optically readable information present on a carrier in the shape of a disc or tape may be read out by means of light emitted by a light source mounted at one side of said carrier, by focusing the light on a small area of the carrier and receiving the light reflected by the carrier by a detector mounted at the same side of the carrier. The carrier is provided with an information carrying structure on that side at which the light source and the detector are present upon reading out. In this structure there are provided fields differing in an optically detectable manner. For instance, these fields may have different reflection coefficients for light of a predetermined wave length, or for light of a predetermined direction of polarization. The information carrying structure may also consist of alternately higher and lower reflecting fields. The difference in height between these two types of fields may amount to about 1/4 of the wave length of the light by which the information is read out. A light beam reflected by a lower field then covers a path which is longer by half a wave length than that of a light beam reflected by a higher field, so that these two light beams are consequently in opposite phase and extinguish one another. When a focused light beam strikes a part of the surface of the carrier consisting partially of a higher field and partially of a lower field, the detector will detect no or but little reflected light. When contrary thereto a focused beam strikes a part of the surface of the carrier solely comprised of a higher (or a lower) field, the detector will detect a considerable quantity of reflected light. In the known carriers the main part of the surface of the tape is normally covered by higher fields, while at least one dimension of the lower fields is in the order of magnitude of 1 .mu.m. It is obvious that dust particles always present in the air may cause a highly disturbing effect if they precipitate onto the information carrying structure. Therefore the information carrying reflecting side of the carrier is usually coated with a protective transparent layer.
The provision of such a protective transparent layer is, however, not necessary if the information carrier is enclosed within a dustproof case.